The invention relates to a method and an apparatus for qualifying optics of a projection exposure tool for microlithography, and to a projection exposure tool comprising this type of apparatus. Furthermore, the invention relates to a sensor module for interferometric wavefront measurement on optics of a projection exposure tool for microlithography.
This type of projection exposure tool has different optics modules, including an illumination system for illuminating a mask with exposure radiation and a projection objective for imaging object structures of the mask onto a substrate in the form of a wafer. Depending on the exposure wavelength, the optics modules have different numbers of mirror elements. When using EUV radiation with a wavelength of e.g. 13.5 nm, mirror elements are used for all of the optical elements of the exposure optical path. The mirror elements optimised for the reflection of EUV radiation have a coating which generally consists of a multilayer structure. The EUV radiation penetrates deeply into the multilayer structure, parts of the EUV radiation being reflected on each individual layer.
Before placing in operation the projection exposure tool and regularly during operation it is necessary to qualify the optical properties of the individual optics modules. The imaging properties of the projection exposure tool often deteriorate during operation of the tool. There can be many causes for this, for example there can be degradation of the mirrors, misalignment of the mirrors etc. The problem then often arises that the cause of such deterioration can not be isolated reliably.